GMC 2007 Sierra - Manual (Page 141)

See “Range Selection Mode” later in this section.
PARK (P): This position locks your rear wheels. It
is the best position to use when you start your
engine because your vehicle cannot move easily. If
your vehicle has the Allison Transmission
® , with the
Power Take-Off (PTO), use PARK (P) when
starting. The Allison Transmission
® has a park pawl
that is designed to hold the weight of commercial
medium duty trucks. Therefore, the Allison
Transmission
® uses larger parts than other
automatic transmissions. These larger parts may
sometimes make a clunking noise when the
selector lever is moved from DRIVE (D) or
REVERSE (R) to PARK (P). This is normal and
there is no damage occurring during these shifts.
When parked on a hill, especially with a heavy load,
you may notice an increase in the effort to shift out
of Park. See Torque Lock (Automatic Transmission)
under Shifting Into Park (P) (Automatic
Transmission) on page 170 for more information.
{ CAUTION:
It is dangerous to get out of your vehicle
if the shift lever is not fully in PARK (P)
with the parking brake firmly set. Your
vehicle can roll.
Do not leave your vehicle when the engine
is running unless you have to. If you have
left the engine running, the vehicle can
move suddenly. You or others could be
injured. To be sure your vehicle will not
move, even when you are on fairly level
ground, always set your parking brake
and move the shift lever to PARK (P).
See Shifting Into Park (P) (Automatic
Transmission) on page 170. If you are
pulling a trailer, see Towing a Trailer on
page 429.
Allison Transmission ®
141